1. Field of the Invention
The present invention relates to a full color flat display panel by using an organic electro-luminescent(EL) device, a manufacturing method thereof and a driving circuit of the organic EL device.
2. Description of the Related Art
Lately, the flat display industry has been remarkably developed. In particular, an organic EL array is gradually gaining interest as an image source in a direct display or a virtual display since the organic EL array can generate a relatively large amount of light and a display adopting the organic EL array can be used under various surrounding conditions.
In other words, the organic EL array can emit light in a sufficient amount so as to be used as the display under the various surrounding conditions ranging from those with no or little amount of light to those with sufficient amount of light.
Also, the organic EL array can be manufactured in a low price, and variously applied from a very small size of under 1 inch up to a considerably large size of tens of inches.
Further, the organic EL array provides a very wide range of view angle.
An example of the organic EL array as a small sized article is applied to portable electronic articles such as a pager, a cellular phone and a portable phone.
Such an organic EL device is comprised of a first electrode layer, an electron transporting layer, a light emitting layer, a hole transporting layer and a second electrode layer.
Here, light can be emitted in one or both directions of an electrode, and the most efficient EL device has one transparent electrode layer in a light emitting side.
Also, one the most widely used transparent electrodes is made of an indium-tin oxide(ITO), which is deposited on a transparent substrate such as a glass panel.
However, the major problem of the organic EL device is the connection capacitance, which includes the capacitance within the device composed of the material and the electrode and the capacitance from column and line electrodes in an array structure.
In other words, since the EL device is driven by a current unlikely to a liquid crystal display(LCD) which is driven by a voltage, an initially supplied current is used for charging the connection capacitance when the organic EL device is driven in the array structure.
Therefore, if the connection capacitance increases as the array increases or the device is enlarged, a larger amount of current should be supplied for the initial charge.
Also, the resistance of an anode line and a cathode line in the array structure gives a very important influence not only to response features of the device but also to the whole power.
In other words, a time for charging the capacitance or an RC time is influenced not only from the size of the capacitance but also to the resistance connected to the capacitance so that the response speed of the device is considerably influenced also as the resistance size of anode and cathode lines increases.
Further, the transparent electrode layer is made of a high resistive material thereby increasing such a problem.
Therefore, the connection capacitance and the electrode layer having high resistance of the organic EL device hinder producing the EL device in a large array structure.
In order to reduce such an influence, the anode and cathode lines can be made of a metal excellent in conductivity and low in resistance to reduce the line resistance in the anode and cathode lines thereby improving the response features of the device and simultaneously to reduce the voltage loss in the line resistance thereby lowering a drive voltage and reducing power consumption.
However, in a delta shaped array structure, such line resistance can be a more serious obstacle due to the fine cathode line. In other words, in a full color device structure, the voltage and current ratio among R, G and B pixels is 3:6:1 for expressing a white light. Therefore, the G(Green) pixel requires a smaller amount of voltage and current than the B(Blue) pixel in expressing the same value of luminance.
FIG. 1 shows a structure of RGB stripe-type pixels, and the current and voltage features in a, a′ and a″ positions of an A material which composes one of the RGB pixels and b, b′ and b″ positions of another B material which composes another one of the RGB pixels.
As can be seen in FIG. 1, the RGB pixels have their own physical properties different from one another so that the current-voltage features may be different in this case.
Referring to FIG. 1, the current-voltage features of the B material is better than those of the A material.
As an example, the A material can be regarded as the R(Red) pixel, and the B material can be regarded as the G or B pixel.
This can be varied according to the properties of materials, in which the A material can be regarded as having the poorest properties.
Also, referring the current-voltage features of each material, if the device has an array structure, there is a difference in the voltage-current features as in a, a′ and a′ and a″ even in the same material due to the effect of line resistance observed along the anode line and along the cathode line.
The properties of the material like this increase the voltage applied to the line resistance of the anode and the cathode such as b, b′ and b″ and a, a′ and a″ to cause the increase of the drive voltage.
In particular, in the case of the A material (for the R pixels for example) which is poor in the current-voltage features, a higher current is required to obtain the white light thereby causing a voltage drop due to line resistance to be more serious.
Here, if the RGB pixels do not have their own power, the driver power should be determined according to a material having the poorest current-voltage features so that the drive voltage in another material having better current-voltage features is elevated thereby incurring overall power loss.
In driving the circuit in practice, the drive voltage is determined according to the pixel having the highest voltage. If the G light emitting pixels are assumed to have the luminance v. current features at least two times better than the R pixels, the drive voltage should be determined by the R pixel so that the drive voltage of the R pixels increases in respect to the G pixels thereby incurring power loss.
As a method of solving this case, the drive voltage is determined different for each of the RGB pixels to reduce power consumption.
However, when each of the RGB pixels is applied with a different voltage from one another, a reverse voltage should be applied to prevent crosstalk according to the features of the organic EL device. Here, the reverse voltage should be applied so that a positive voltage applied to the device does not exceed the threshold voltage.
Therefore, since the reverse voltage is applied according to the drive voltage of the R light emitting pixels if the R light emitting pixels have the highest drive voltage, the higher reverse voltage should be applied as the drive voltage of the R light emitting pixels is stepped up. So, many problems are caused in using the reverse voltage.
Meanwhile, FIG. 2A is a sectional view of the full color organic EL display device driven by the driving circuit of FIG. 1.
In manufacturing the organic EL display device, a shadow mask is used to form the RGB light emitting pixels having the optimal luminous efficiency as shown in FIG. 2A.
Also, the shadow mask as above is also used in a line method as shown in FIG. 2B, and in a method of arranging the pixels into a delta shape as shown in FIG. 2C.
In other words, as shown in FIG. 2A, anode lines 2—2 (only one is shown) are formed on a glass 1, and partitions 7 are formed before forming cathode lines in the organic EL display device. Then, red, green and blue emitting material layers 8-1, 8-2 and 8-3 are formed by using a shadow mask 9 followed by forming cathodes 10 for forming cathode lines in the front surface.
However, a cell array structure of the full color organic EL device of the related art described hereinbefore has the following problems:
In arranging the pixels according to the line method or the delta shape, the RGB light emitting pixels are sized almost the same so that the R pixel relatively poor in luminance and luminous efficiency is not properly expressed thereby degrading the texture.
Also, the opening ratio is lowered and ITO lines for connecting the light emitting pixels are thinned and elongated to increase resistance so that the uniformity across the screen is degraded and the drive voltage is elevated.